There has been an increasing preference for a rubber material excellent in fracture resistance, wear resistance and crack growth resistance in recent years due to pursuit of better durability of a tire responding to social demand for saving energy and resources. Further, a recent rising price of natural rubber necessitates development of synthetic rubber having durability equivalent to that of natural rubber.
There have conventionally been attempts to improve strain-induced crystallinity of synthetic polyisoprene by increasing the cis content thereof in order to enhance durability of the polyisoprene (refer to PTL 1 and PTL 2, for example). However, use of synthetic polyisoprene as a rubber composition in place of natural rubber requires synthetic polyisoprene to be blended with other polymer components because wear resistance and fracture resistance of synthetic polyisoprene are not as high as those of natural rubber. Relevant respective polymers are therefore charged into and mixed by a mixer in general when synthetic isoprene and other polymers are mixed. There arises a problem, however, in this regard in that polymers are not dispersed in a satisfactory manner by such mixing as described above and a resulting polymer composition may not have physical properties as desired.